Treat that rust before it gets worse. The best stuff to use, I would consider it better than POR-15 due to its superior adhesion and little prep requirements(enabling you to truly 'paint over rust' in hard to reach rusty areas like the inside of the front engine bay drains) is Rust Bullet. That stuff is pretty good but has a short shelf life after you open it. One drawback is it's silver in color which makes it stick out like a turd in a punchbowl. Have to use some body colored spray paint over it but otherwise the stuff is tough. My repair is 2 years old and doesn't show the slightest hint of getting worse.

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'02 BMW 325i
'85 300D 450k
'93 190E 2.6 170k(killed by tree)
'08 Ducati Hypermotard 1100S 6k
'06 Ducati S2R800 14k(sold)